FIG. 1 shows a conventional sealing device for a variable geometry turbocharger. The turbocharger comprises turbine and compressor housings 1 and 2 integrally assembled via a bearing housing 3 with fastening bolts 3a and 3b, turbine and compressor impellers 4 and 5 in the housings 1 and 2 being coupled together with a turbine shaft 7 rotatably supported in the bearing housing 3 by a bearing 6.
As shown in FIG. 2 which is an enlarged view of a portion II in FIG. 1, the bearing housing 3 has, on its side adjacent to the turbine housing 1, an exhaust nozzle 10 (shroud) with a plurality of radial vanes 9 arranged circumferentially equidistantly between turbine- and bearing-housing-side exhaust introducing walls 9a and 9b for guidance of fluid (exhaust gas) introduced in a scroll passage 8 of the housing 1 to the turbine impeller 4, the exhaust nozzle 10 being sandwiched by the turbine and bearing housings 1 and 3 and fixed with the fastening bolt 3a in FIG. 1.
In FIG. 2, reference numerals 10a and 10b denote vane shafts extending through the turbine- and bearing-housing-side exhaust introducing walls 9a and 9b of the exhaust nozzle 10 to rotatably support the vanes 9, the respective vanes 9 being supported on opposite sides thereof by the vane shafts 10a and 10b. Alternatively, the vanes 9 may be supported on the opposite sides thereof with the turbine-housing-side vane shafts 10a being short shafts not extending through the exhaust introducing walls 9a but embedded in bearing-housing-side surfaces of the walls 9a. Alternatively, without the turbine-housing-side vane shafts 10a, the vanes 9 may be supported in a cantilever manner only by the vane shafts 10b extending through the bearing-housing-side exhaust introducing wall 9b. 
In FIG. 1, reference numeral 11 denotes a positioning pin used in assembling of the exhaust nozzle 10; 12, a scroll passage in the compressor housing 2; and 13a, 13b, 13c and 13d, a link mechanism for adjusting an opening degree of the vanes 9 via the vane shafts 10b. 
The turbine housing 1 providing the scroll passage 8 is formed with a nozzle facing portion 14 which faces the turbine-housing-side exhaust introducing wall 9a of the exhaust nozzle 10, an annular gap 15 being formed between the nozzle 10 and the nozzle facing portion 14 to extend radially and open to the scroll passage 8. The exhaust introducing wall 9a constituting the exhaust nozzle 10 has an inner end extending as an extension 17 along the turbine impeller 4 toward a shoulder 16 formed on an inner periphery of the nozzle facing portion 14, which brings about a gap 15′ between the extension 17 and the shoulder 16 opening to the inner periphery of the portion 14 and communicating with the gap 15.
The gaps 15 and 15′ are inherently unnecessary, but are provided for countermeasure to, for example, any thermal deformation of the turbine housing 1 between hot and cold states and any dispersion in accuracy between parts to be assembled.
However, when the gaps 15 and 15′ are present, the gas in the scroll passage 8 may disadvantageously leak through the gaps 15 and 15′ from the higher to the lower pressure side, and the gas leakage substantially varies the performance of the turbocharger on a lower velocity side, leading to a problem such as unstable engine performance.
Thus, it has been conventionally proposed as shown in FIG. 2 that an annular groove 18 is formed on an outer periphery of the extension 17 of the exhaust nozzle 10, and sealing piston rings 19 and 20 generally used for a piston of an engine are fitted into the groove 18 to provide a sealing device, so that outer peripheries of the piston rings 19 and 20 are brought into close contact with an inner periphery of the shoulder 16 due to resilience of the rings 19 and 20. Thus, the gas leakage from the gaps 15 and 15′ is prevented and any thermal deformation is absorbed.
General state-of-art relevant to the sealing device for the turbocharger shown in FIGS. 1 and 2 is disclosed, for example, in Patent Literature 1.